I agree with the people who say the seller has done your job for them by leaving their own negatives. Unfortunately they left them on your ebay page, but nobody with any sense will consider it your bad.
What amuses me is that they accuse you of penny pinching - you, the person who offered to pay more for a box, and you, the person who did bid on their item - and yet they admit themselves "we only sold this for 99p so we're going for cheap and not giving a box". Talk about pot and kettle. Irony doesn't even begin to describe it.
It's not your fault if nobody else bid on the item and upped the bid to a higher amount. Whether they had other ponies or not is not the point. The seller is responsible for shipping the item and the buyer is responsible for paying the shipping quote that the seller gives in good faith that it will be shipped properly. In this instance, you did nothing wrong. Taking the feedback off would hide the way they acted and the comments will appear on their page if anyone looks at the neutral. If their comment can't be removed, nor can yours (and you can bet they already tried, hence the anger). Anyone looking at their items will see that for what it is, and they may well lose sales for it.
I would call that justice.